The above-mentioned related U.S. patent application U.S. Ser. Nos. 08/117,924, 08/116,793, 08/195,659, and U.S. Pat. No. 5,444,295 disclose a substrate subassembly that includes a metallized thin rectangular ceramic substrate having two semiconductor chips thereon and an integral thin metal connector tab extending from one of the metallized surfaces. In a preferred embodiment, one surface of a second ceramic wafer is bonded to the metallized wafer surface that has the tab. The opposite surface of the second wafer is metallized, and also has a tab extending therefrom. More specifically, the wafers are of beryllium oxide and the metallizations are copper foil sheets bonded to the wafer surface. The tabs are integral extensions of the copper foil sheets beyond the wafer edge.
The bonded copper foil sheets become copper foil layers in a composite substrate. The copper foil layers are bonded to the opposed major surfaces of the beryllium oxide wafers by direct copper bonding. Direct copper bonding is a known technique by which copper oxide is used to bond copper metal to a ceramic.
The semiconductor chips are bonded to one of the copper foil layers, preferably by soldering, to form a resultant substrate subassembly. As indicated above, the copper foil layer to which the chips are soldered, has an integral tab portion that overhangs the beryllium oxide wafer edge. The tab projects out from the beryllium oxide wafer a short distance, but a distance long enough to permit easy attachment to a cooperating terminal member for the region of each chip contacting the copper foil layer. The tab, thus, provides an integral connector portion between that copper foil and the terminal member. In the preferred five layer substrate subassembly, upper surface portions of the chips are "wire bonded" to the top copper foil layer. The tab on the top copper foil layer connects to a second terminal member, to provide low resistance communication between the second terminal member and the upper surfaces of the chips.
The substrate subassembly, as well as its manufacture and function, is the subject matter of U.S. patent application Ser. No. 08/118,112. Use of substrate subassemblies in modules is described and claimed in U.S. patent application Ser. No. 08/117,924. A coaxial single switch module having such substrates is the subject matter of U.S. patent application Ser. No. 08/116,793. A linear dual switch module having such substrates is the subject matter of U.S. patent application Ser. No. 08/118,112. A coplanar linear dual switch module having a special composite terminal subassembly is the subject matter of U.S. patent application Ser. No. 08/195,659. A special electron beam or laser beam welding technique, for bonding the copper foil tabs (of the substrate subassembly) to a terminal member, is the subject matter of U.S. patent application Ser. No. 08/233,572. The subject patent application is an improvement on U.S. patent application Ser. No. 08/116,793.
Before discussing this application's improvements over U.S. patent application Ser. No. 08/116,793, basic features of the latter application shall be reviewed.
U.S. patent application Ser. No. 08/116,793 focuses its description on packaging insulated gate bipolar transistors (IGBTs). IGBTs are extremely attractive semiconductor devices for power applications. An IGBT is more attractive than a power-type insulated gate field effect transistor (IGFET), which is popularly referred to as a MOSFET. An IGBT can handle both high voltages and high currents with small die size and with relatively low "on" resistance. In addition, an IGBT can be switched rapidly, making IGBTs potentially useful as switches in a three phase inverter for a high power alternating current motor application.
On the other hand, the high current density capability and low "on" resistance of the IGBT also present new challenges. The possibility of device failure is aggravated when the IGBT is handling high power. By high power, we mean current densities above about 135 amps per square centimeter of active chip area, at hundreds of volts. By high frequency switching we mean on/off frequencies above about 18 kilohertz, as for example 30 kilohertz. As might be expected, significant impedance, material and mechanical problems are encountered in handling such power at high frequencies and low resistances. This is especially true for a high power/high frequency module, in which several such IGBTs are electrically paralleled. Heretofore, the foregoing problems have been so difficult that not many high power/high frequency IGBT modules have been commercially manufactured. Those that were made thus far, have been made in relatively low volume, where each module could be individually specially crafted.
The prior U.S. patent application Ser. No. 08/116,793 describes a high frequency/high power coaxial-type module that is capable of being manufactured on a commercial production basis. By commercial production basis, we mean production volumes such as used in the automotive industry. The high power/high switching frequency IGBT modules of the prior U.S. patent application Ser. No. 08/116,793 (5922-00010; H-172190) have high efficiency and high durability, but are still economically manufacturable in automotive-type high volumes.
The prior U.S. patent application Ser. No. 08/116,793 suggests integrating module terminal members into a premolded subassembly for a single switch, and that coplanar contacts would be desirable. The subject patent application takes that suggestion further. It provides a unique pre-molded subassembly of three concentric members for a double switch, not of two members for a single switch. In the subject three member subassembly, the members are concentric and have interdigitated finger flanges. In addition, the interdigitated finger flanges of our unique terminal member subassembly have coplanar tips that comprise coplanar terminal contact areas for connection with coplanar connector tabs on the substrate subassemblies in the module. Still further, the premolded terminal subassembly is pretestable, and easily assembled into the module. This increases speed and simplicity of assembly. It also allows an increase in both production speed and yield. Also, our unique premolded terminal subassembly is designed to allow module pretesting at full operating power before final packaging steps are completed. This latter pretesting allows the discarding or repair of substandard modules at a readily fixable stage, and before more value is added. Discarding or repair of module parts before assembly, or of the module itself in early stages of assembly, of course, helps reduce manufacturing costs and overall device quality and durability.
Further, with coplanar terminal contact areas and coplanar substrate subassembly coplanar tab ends for all switches in the module, all terminal connections for any switch or group of switches can be made in a single welding operation. This allows the electron and laser beam welding concepts of the above-mentioned U.S. patent application Ser. No. 08/233,572 to be more fully utilized. This additional utilization, provides a still further manufacturing advantage. We refer to our unique subassembly of three concentric center terminals for a double switch module as triaxial. We believe that our triaxial terminal member subassembly, and our methods of making the subassembly and incorporating it into the module, are quite applicable to high volume and demanding applications. Such applications would include, for example, automotive electric vehicle inverter commercial production applications.
Moreover, our unique triaxial terminal member subassembly helps provide a module of high efficiency and high durability, but economically manufacturable at high volumes.